The major thrust of this proposal is to determine the extent of capillary recruitment in the control of cerebral blood flow (CBF). The emphasis will be on the function of the control system as it is effected in the microvessels of the cerebral cortex. Both the temporal and spatial qualities of the capillary recruitment control system will be studied by altering physiological variables that affect CBF. The connection between oxygen cycles, CBF microheterogeneity or the columnar distribution of CBF, and the vascular architecture of the cortex all support the concept of a CBF control system based upon the capillary recruitment model. The purpose of this proposal is to study and define this hypothesized control system with newly developed experimental techniques. The quick snapshot of CBF that is available with the current autoradiographic techniques and the introduction of a vascular stain which can be used at the same time as a CBF study will elucidate the functioning and mechanisms of this hypothesized control system. CBF will be measured over two different intervals in the same animal using double radionuclide autoradiography. Cerebral blood volume will be determined in conjunction with CBF using the double label autoradiographic techniques. The physiological variables known to influence CBF (PaCO2, PaO2 and mean arterial blood pressure) will be altered to define the limits of the columnar structure of CBF and the relationship of this structure to capillary recruitment. The dysfunction of this hypothesized control system will be studied in ischemia, hypoxia and recovery from complete ischemia. This investigation is centered on the mechanistic and functional aspects of CBF control. The implication and applications to pathology will also be addressed.